fix: add prettier as direct devDependency

prettier is needed by the format:check script but was only available
as a transitive dependency of @kinvolk/headlamp-plugin.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

package.json

@@ -31,6 +31,7 @@
     "@testing-library/user-event": "^14.5.2",
     "eslint": "^8.57.0",
     "jsdom": "^24.0.0",
+    "prettier": "^3.0.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-router-dom": "^5.3.0",